Field of the Invention
Embodiments of the present application relates to the art of a control system for a hybrid vehicle having a selectable one-way clutch that halts a rotation of a predetermined rotary member.
Discussion of the Related Art
Japanese Patent Publication No. 5120650 describes a hybrid vehicle having a clutch selectively providing a connection between an engine and a planetary gear unit serving as a power distribution device, and a brake selectively halting a rotation of an input shaft connected to the engine. According to the teachings of Japanese Patent Publication No. 5120650, in the planetary gear unit, a first rotary element is connected to the input shaft, a second rotary element is connected to a first motor-generator, and a third rotary element serves as an output element. A second motor-generator is disposed on a power transmission route between the output element and drive wheels. The hybrid vehicle taught by Japanese Patent Publication No. 5120650 can be powered by the first motor-generator and the second motor-generator by supplying electric power to those motor-generators while releasing the clutch and halting the input shaft by the brake. In this situation, when restarting the engine, the engine is started by the first motor-generator while releasing the brake and connecting the input shaft to the engine through the clutch.
JP-A-2008-267866 describes a viscosity detection apparatus that detects viscosity of lubricant based on a rotational resistance of an impeller at a site where a viscosity sensor is installed. JP-A-2013-086585 describes a drag torque acting between an inner clutch plate and an outer clutch plate of a drive force transmission device through lubricant.
An operating mode of the conventional selectable one-way clutch is switched between an engagement mode in which the selectable one-way clutch is enabled to transmit torque in one direction, and an overrunning mode in which the selectable one-way clutch is not allowed to transmit torque in both directions. The conventional selectable one-way clutch is comprised of a pocket plate having a plurality of struts, a notch plate having a plurality of notches individually engaged with the struts, and a selector plate having a plurality of apertures for letting though the struts. The pocket plate and the notch plate are axially opposed to each other, and the selector plate is interposed between the pocket plate and the notch plate while being allowed to rotate relatively therewith. The brake taught by Japanese Patent Publication No. 5120650 is adapted to selectively halt a forward rotation of the input shaft. To this end, the selectable one-way clutch may be used in the hybrid vehicle taught by Japanese Patent Publication No. 5120650 instead of the brake.
In order to reduce friction between the strut and the selector plate or the notch plate and friction between those plates, lubrication oil is applied to the selectable one-way clutch. However, a drag torque is generated between the plates depending on viscosity of the oil. Specifically, viscosity of oil is increased with a reduction in a temperature thereof, and such drag torque is increased with an increase in the viscosity. As described, the selectable one-way clutch may be used in a hybrid vehicle to selectively halt a rotation of the input shaft or the engine connected thereto. However, if the oil viscosity is high, the selector plate may be rotated unintentionally when allowing the input shat or the engine to rotate by a drag torque resulting from a rotation of the input shaft or the engine, and consequently, the selectable one-way clutch may be brought into engagement unnecessarily. If the selectable one-way clutch is thus brought into engagement undesirably when restarting the engine, the engine cannot be rotated by the motor.